Global navigation satellite systems (GNSS) are broadly defined to include GPS (U.S.), Galileo (proposed), GLONASS (Russia), Beidou (China), IRNSS (India, proposed), QZSS (Japan, proposed) and other current and future positioning technologies using signals from satellites, with or without augmentation from terrestrial sources. Information from GNSS is being increasingly used for computing a user's positional information (e.g., a location, a speed, a direction of travel, etc.).
In GNSS, multiple satellites may be present, with each transmitting a satellite signal. A received signal at a GNSS receiver contains one or more of the transmitted satellite signals. To obtain the information from the respective transmitted signals, the GNSS receiver performs a signal acquisition/tracking procedure. More specifically, the GNSS receiver searches for the corresponding transmitted satellite signals in the received signal and, then locks onto them for subsequent tracking of the corresponding satellites to receive the satellite information.
When a GNSS receiver is turned on, it searches for satellite signals that match a known PN (pseudorandom noise) code and a carrier frequency (acquisition phase). The carrier frequency of the satellite signal and the PN code phase (phase of PN code) perceived by the GNSS receiver may vary over time due to doppler effect, which is caused by relative motion between the transmitting satellite and the GNSS receiver, and also drifts in the frequency of the clock used by the GNSS receiver to sample the PN code. A match of a known PN code and a carrier frequency in a received signal identifies the transmitting satellite. The GNSS receiver tracks the carrier doppler frequency and PN code phase of the satellite signal after they are acquired with the help of frequency locked loop (FLL), delay locked loop (DLL) and other GNSS receiver tracking circuits (tracking phase).
Typical open-sky GNSS satellite signal power level is −130 dBm. However, the GNSS satellite signal power level is less than −160 dBm while indoors and under tunnels. In good GNSS satellite signal conditions, the GNSS receiver would be able to track the GNSS satellite signal. However, if a user suddenly accelerates or the GNSS satellite signal is obstructed because of building, tunnels, sub-ways etc, then the GNSS receiver would lose track of the satellite signal. In such a case, the GNSS receiver has to again undergo the satellite signal acquisition process (acquisition phase). Modern GNSS receivers use an intensive hardware and firmware to first acquire the carrier frequency and PN code sampling phase (also called code phase) of the various visible GNSS satellite signals, and a less intensive hardware and firmware to then track the doppler effect caused variations after initial acquisition. Thus, acquisition of the GNSS satellite signal is more power intensive process than the tracking of the GNSS satellite signal. Thus, a GNSS receiver is required that can efficiently track the GNSS satellite signal even at low GNSS satellite signal power levels.